1. Field of the Invention
The present invention relates to a charge transfer device, and more particularly to a charge coupled device (hereinafter is referred to as CCD).
2. Description of the Related Art
The charge transfer device, in particular the CCD has many technical advantages such as having rather small size and simple configuration; and therefore, intensive research and development thereon are being made. Furthermore in recent years, because high quality reproduced picture is demanded, an increase in transfer frequency of CCD devices is further demanded. In conventional CCDs', plural horizontal CCDs' are provided such that transfer frequencies are halved in order to prevent complication circuit design and increased power consumption. Such conventional art is disclosed, for instance, in the gazette of the Japanese published unexamined patent application Sho 60-189966 (Tokkai Sho 60-189966). The art alleges the advantage that the frequency of a transfer signal is halved, the density of integration of a horizontal CCD is halved and the power consumption is low.
FIG. 10(a) shows a plan-view configuration of a portion of a prior art horizontal CCD apparatus in Tokkai Sho 60-189966. FIG. 10(b) is an overall circuit block diagram of a general CCD apparatus which includes the horizontal CCD apparatus of FIG. 10(a), wherein a float diffusion layer (FD) region 30 is provided at the lower end of the HCCDs' 27 and 28. Vertical CCDs' (hereinafter are referred to as VCCDs') 31, . . . , 31 are connected vertically to the EICCDs' 27. The device of FIG. 10(a) has a pair of horizontal CCD (hereinafter referred to HCCD) 27 and 28 with a transfer gate 29 therebetween. HCCDs' 27 and 28 are connected through horizontal gates 25 and 26 to common floating diffusion layer (hereinafter is referred to as FD) 30. The horizontal gates 21, 22, 23 and 24 constitute a two-phase driving CCD, and charges are temporarily stored underneath the wider horizontal gates 22 and 24 and transferred stepwise in X-direction therebetween. The horizontal gates 25 and 26 operate as a two-phase driving CCD similarly to the horizontal gates 21, 22, 23 and 24. Transfer gate 29 is formed by a first polysilicon layer, the horizontal gates 22 and 24 are formed by a second polysilicon layer formed thereafter and the horizontal gates 21 and 23 are formed by a third polysilicon layer formed further thereafter. Reset signal .o slashed.R is applied to the reset gate 35. At a point of time T=t0, charge transfer from a vertical CCD (not shown) to HCCD 27 is over. At and after a time point t1, the signal .o slashed.tG applied to transfer gate 29 is ON, and the charges in through-channel A Or FIG. 10(a) is transferred through transfer gate 29 to HCCD 28. At that time, charges in interrupted-channel B remain in parts in HCCD 27, obstructed by isolation region 40. Isolation regions 40 are made by ion implantation or by thick insulation regions. At time point T2 when the signal .o slashed.H12 turns OFF, the charges in through-channel A of the HCCD 27 are all transferred through portions under transfer gate 29 to portions of HCCD 28. Furthermore, since the signal .o slashed.H11 is ON at that point in time, the charge in the B region remains still under horizontal gate 22 in the HCCD 27. At a time point t3 when the signal .o slashed.VG turns OFF, the charges under transfer gate 29 are all transferred to portions under horizontal gate 22 in HCCD 28.
In the above-mentioned manner, the charge transfer is made either to the HCCD 27 or to the HCCD 28.
The operation of the conventional CCD device in FIG. 10(a) is as follows:
When a signal charge is transferred as shown by arrows 31, 31, . . . to portions underneath horizontal gates 22 and 24, this charge is transferred, by the operation of transfer gate 29, to portions underneath horizontal gate 22 in HCCD 28. Regions 40, 40, and 38 are isolation regions, and are formed by impurity diffused channel stoppers or thick oxide films. From terminals 32 and 33, horizontal transfer pulse signal .o slashed.H11 is applied to horizontal gates 21 and 22, and also horizontal transfer pulse signal .o slashed.H12 is applied to horizontal gates 23 and 24, respectively. And thereafter, the charges are transferred through HCCDs' 27 and 28 and led, through combining gates 25 and 26, to output gate 34, and finally transferred to the FD 30.
FIG. 11 is a time chart showing operation signals of the apparatus of FIG. 10(a). Signal .o slashed.tG is applied to the transfer gate 29. Signal .o slashed.AG is applied to the combining gates 25 and 26 through a terminal 39. And after dividedly being transferred to the two HCCDs' 27 and 28, the charges in the HCCD 27 and in the HCCD 28 are alternatively transferred towards the FD 30 by means of signals .o slashed.H12 and .o slashed.H11. Thus the frequencies of the transfer driving signals .o slashed.H12 and .o slashed.H11 are halved in comparison with older constructions using a non-divided HCCD.
However, the prior art device shown in FIG. 10(a) and FIG. 11 has the following problems:
FIG. 12(a), FIG. 12(b) and FIG. 12(c) show potential distribution in this prior art device. As shown in FIG. 13(a) and (b), the pitches of a gate configuration in the x-direction become short as the degree of integration increases; and in general, a narrow-channel effect, which makes the potential low, appears when the gaps between gates becomes short. The case of a narrow-channel effect appearing in the prior art is elucidated.
Because of problems with the manufacturing process, a gate has microscopic irregularity, and hence gaps between gates can have delicate variations. Potential distribution in the Y-direction is influenced by a narrow-channel effect such that the potential becomes higher and lower as the gate gaps become wider and narrower, respectively. As a result, potential irregularity is produced in the Y-direction depending on the variation of gate gaps. Hence the transfer loss shown as hatched through-channel A in FIG. 12(a) is produced when the charge is transferred from HCCD 27 to HCCD 28. Defining the amount of the transfer loss as .alpha., a next stage of HCCD 27 is added by the amount .alpha. to the actual charge, and the actual charge transferred to the HCCD 28 is less by the amount .alpha. from the ideal amount. Such amount .alpha. results in FPN (fixed pattern noise) to deteriorate the picture quality. Such FPN is produced not only by the irregularity of HCCD 27, but may be produced also, as shown by FIG. 12(b) when the gap .gamma. between the isolation regions in the transfer gate 29 is narrower than the width W (in the X-direction) of HCCD 27, or as shown by FIG. 12(c) when the potential recess is formed in the transfer direction X of the HCCD 27 and 28. In such cases, as shown in FIG. 12(c), a raised part B of potential is formed, and therefore the charge transfer from the HCCD 27 to the transfer gate 29 is not made sufficiently.
Even when both opposing sides along an isolation gap are formed quite parallelly, if the parallel sides are extending very long, an irregularity along the parallel sides will form. Or alternatively, the charges sometimes are not transferred completely, making reverse flow at the last part or beyond midway part of the transfer, thereby resulting in transfer loss similar to the partition noise.
As has been elucidated, the CCD device of the conventional configuration has the problem of providing considerable deterioration of picture quality due to FPN and the like, which are caused by charge transfer losses generated at various parts, and induced by narrow-channel effects due to the high degree of integration. Hitherto, severe and difficult measures have been taken not to produce such narrow-channel effects, in the technologies of designing and manufacturing process. However, when a higher degree of integration is demanded, a narrow channel effect becomes an essential problem to be overcome rather than one to be avoided or averted.
With regard to isolation region 40, another problem of this prior art device is elucidated with reference to FIG. 13(a) and (b) showing plan views of the pertinent portion of the device. Generally isolation region 40 is formed by ion-implantation of p-conductivity type atoms. However, even though the isolation region 40 is formed rectangular or square as shown in FIG. 13(a), the actually formed isolation region is diffused to the region shown by the broken lines. This is mainly due to thermal diffusion during heat treatment steps, and thermal diffusion is not easily controllable, hence, thermal diffusion cannot be avoided. When an external voltage applied to the CCD is zero, the potential is as shown in FIG. 13(b), wherein the chain lines show equipotential lines and the batched parts show regions of potentials above the equipotential. In a conventional device, the equipotential lines along the transfer gate 29 are not straight, but gently meander. Therefore, when the actual isolation region 40 diffuses considerably outside the designed regions 40, 40, . . . , thereby to form the meandering equi-potential lines as shown in FIG. 13 (b), the virtual or effective regions of HCCD's 27 and 28 are not straight rectangles but irregularly formed, thereby lowering the transfer efficiency in the direction towards FD30.